This invention relates to a method and apparatus for compensating for distortion in a radio apparatus and, more particularly, to a distortion compensation method and distortion compensation apparatus for suppressing non-linear distortion of a transmission power amplifier in a radio apparatus and reducing leakage of power between adjacent channels.
Frequency resources have become tight in recent years and in wireless communications there is growing use of high-efficiency transmission using digital techniques. In instances where multilevel amplitude modulation is applied to wireless communications, a vital technique is one which can suppress non-linear distortion by linearizing the amplitude characteristic of the power amplifier on the transmitting side and reduce the leakage of power between adjacent channels. Also essential is a technique which compensates for the occurrence of distortion that arises when an attempt is made to improve power efficiency by using an amplifier that exhibits poor linearity.
FIG. 22 is a block diagram illustrating an example of a transmitting apparatus in a radio according to the prior art. Here a transmit-signal generator 1 transmits a serial digital data sequence and a serial/parallel (S/P) converter 2 divides the digital data sequence alternately one bit at a time to convert the data to two sequences, namely an in-phase component signal (also referred to as an “I signal”) and a quadrature component signal (also referred to as a “Q signal”). A DA converter 3 converts the I and Q signals to respective analog baseband signals and inputs these to a quadrature modulator 4. The latter multiplies the input I and Q signals (the transmit baseband signals) by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90° and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. A frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier 6 power-amplifies the carrier output from the frequency converter 5. The amplified signal is released into the atmosphere from an antenna 7.
In such a transmitting apparatus, the input/output characteristic [distortion function f(p)] of the transmission power amplifier is non-linear, as indicated by the dotted line in (a) of FIG. 23. Non-linear distortion arises as a result of non-linear characteristics, and the frequency spectrum in the vicinity of a transmission frequency f0 develops side lobes, as shown in (b) of FIG. 23, leakage into the adjacent channel occurs and this causes interference between adjacent channels. For this reason, the Cartesian-loop method and polar-loop method, etc., have been proposed as techniques for effecting distortion compensation by feedback and these methods are used to suppress the distortion of power amplifiers.
FIG. 24 is a block diagram of a transmitting apparatus having a digital non-linear distortion compensating function that employs a DSP. Here a group of digital data (a modulation signal) sent from the transmit-signal generator 1 is converted to two signal sequences, namely I and Q signals, by the S/P converter 2, and these signals enter a predistorter 8 constituted by a DSP. As illustrated in FIG. 25, the distortion compensator 8 functionally comprises a distortion compensation coefficient memory 8a for storing distortion compensation coefficients h(pi) (i=0˜1023) conforming to power levels 0˜1023 of the modulation signal; a predistortion unit 8b for subjecting the modulation signal to distortion compensation processing (predistortion) using a distortion compensation coefficient h(pi) that is in conformity with the level of the modulation signal; and a distortion compensation coefficient calculation unit 8c for comparing the modulation signal with a demodulated signal, which has been obtained by demodulation in a quadrature detector described later, and for calculating and updating the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will approach zero.
The distortion compensator 8 subjects the modulation signal to predistortion processing using the distortion compensation coefficient h(pi) that conforms to the level of the modulation signal, and inputs the processed signal to the DA converter 3. The latter converts the input I and Q signals to analog baseband signals and applies the baseband signals to the quadrature modulator 4. The latter multiplies the input I and Q signals by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90°, respectively, and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. The frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and the transmission power amplifier 6 power-amplifies the carrier signal output from the frequency converter 5. The amplified signal is released into the atmosphere from the antenna 7. Part of the transmit signal is input to a frequency converter 10 via a directional coupler 9, whereby the signal undergoes a frequency conversion and is input to a quadrature detector 11. The latter performs quadrature detection by multiplying the input signal by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90°, reproduces the I, Q signals of the baseband on the transmitting side and applies these signals to an AD converter 12. The latter converts the applied I and Q signals to digital data and inputs the digital data to a distortion compensator 8. By way of adaptive signal processing using the LMS (Least Mean Square) algorithm, the distortion compensator 8 compares the modulation signal with the demodulated obtained by demodulation in the quadrature detector 11 and proceeds to calculate and update the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will become zero. The modulation signal to be transmitted next is then subjected to predistortion processing using the updated distortion compensation coefficient and the processed signal is output. By thenceforth repeating this operation, non-linear distortion of the transmission power amplifier 6 is suppressed to reduce the leakage of power between adjacent channels.
FIG. 26 is a diagram useful in describing distortion compensation processing by an adaptive LMS. A multiplier (which corresponds to the predistortion unit 8b in FIG. 25) 15a multiplies the modulation signal (the input baseband signal) x(t) by a distortion compensation coefficient hn−1(p). A transmission power amplifier 15b has a distortion function f(p). A feedback loop 15c feeds back the output signal y(t) from the transmission power amplifier and an arithmetic unit (amplitude-to-power converter) 15d calculates the power p [=x(t)2] of the modulation signal x(t). A distortion compensation coefficient memory (which corresponds to the distortion compensation coefficient memory 8a of FIG. 25) 15e stores the distortion compensation coefficients that conform to the power levels of the modulation signal x(t). The memory 15e outputs the distortion compensation coefficient hn−1(p) conforming to the power p of the modulation signal x(t) and updates the distortion compensation coefficient hn−1(p) by a distortion compensation coefficient hn(p) found by the LMS algorithm.
Reference characters 15f denote a complex-conjugate signal output unit 15f. A subtractor 15g outputs the difference e(t) between the modulation signal x(t) and the feedback demodulated signal y(t), a multiplier 15h performs multiplication between e(t) and u*(t), a multiplier 15i performs multiplication between hn−1(p) and y*(t), a multiplier 15j performs multiplication by a step-size parameter μ, and an adder 15k adds hn−1(p) and μe(t)u*(t). Reference characters 15m, 15n, 15p denote delay units. A delay time, which is equivalent to the length of time from the moment the transmit signal x(t) enters to the moment the feedback (demodulated) signal y(t) is input to the subtractor 15g, is added onto the input signal. The units 15f˜15j construct a rotation calculation unit 16. A signal that has sustained distortion is indicated at u(t). The arithmetic operations performed by the arrangement set forth above are as follows:hn(p)=hn−1(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=hn−1(p)x(t)f(p)u(t)=x(t)f(p)=h*n−1(p)y(t)P=|x(t)|2where x, y, f, h, u, e represent complex numbers and * signifies a complex conjugate. By executing the processing set forth above, the distortion compensation coefficient h(p) is updated so as to minimize the difference e(t) between the transmit signal x(t) and the feedback (demodulated) signal y(t), and the coefficient eventually converges to the optimum distortion compensation coefficient h(p) so that compensation is made for the distortion in the transmission power amplifier.
FIG. 27 is a diagram showing the overall construction of a transmitting apparatus expressed by x(t)=I(t)+jQ(t). Components in FIG. 27 identical with those shown in FIGS. 24 and 26 are designated by like reference characters.
As mentioned above, the principle of digital non-linear distortion compensation is to feed back and detect a carrier obtained by quadrature modulation by a modulating signal, digitally convert and compare the amplitudes of the modulating signal (transmit baseband signal) and feedback signal (feedback baseband signal), and update the distortion compensation coefficient in real time based upon the comparison.
As indicated by a frequency spectrum FS1 in FIG. 28, a phenomenon (frequency asymmetric distortion) occurs in which the actual transmission power amplifier generates unnecessary radiation power that differs between positive and negative frequency regions with respect to a center frequency f0. Further, frequency asymmetric distortion differs depending upon individual differences among devices. The reason for this phenomenon is that the distortion function of a transmission power amplifier depends not only upon the instantaneous value p of input power but also upon the value of input power in the past.
With conventional distortion compensation processing (predistortion), distortion compensation coefficients are updated on the assumption that the distortion function f(p) is dependent solely upon the instantaneous value p of input power. As a consequence, a frequency spectrum FS2 in a case where conventional distortion compensation processing (predistortion) has been carried out becomes as shown in FIG. 28, and a problem which arises is that although a distortion suppression effect is obtained, the suppression effect is not satisfactory.